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FAA AD on turnbuckle corrosion

FAA AD on turnbuckle corrosion

RE: FAA AD on turnbuckle corrosion

The importance of the recommended actions should not be understated. However, the thread title is somewhat confusing.

The FAA has not determined there is an unsafe condition in need of an AD. If anyone is concerned about the impact this could have, keep an eye on the RGL or subscribe to it's notification service. You will see an NPRM well before any regulatory action is implemented. As of yet there is no NPRM.

Keep em' Flying
//Fight Corrosion!

RE: FAA AD on turnbuckle corrosion

Berk... this is disturbing news...

The ATSB and NTSB [attached] report has far more detail than the FAA SAIB.
https://files.engineering.com/getfile.aspx?folder=...

When I reviewed the 'procurement spec' MIL-DTL-781(J) DETAIL SPECIFICATION SHEET TERMINAL, WIRE ROPE, SWAGING, STUD ...

3.3 Material. Unless otherwise specified, all terminals, except NAS1435, shall be made of corrosion resistant steel, 303Se (UNS S30323) or 304 (UNS S30400). ...

... and combined this with the ATSB and NSTB report findings that all the failed/corroded terminals were made from [MF] 303se SStl... NOT 304 SStl...

... and combined this with MIL-STD-1587(E) MATERIAL AND PROCESS REQUIREMENTS FOR AEROSPACE WEAPONS SYSTEMS, TABLE I. Commonly restricted structural materials. where all 303, 303S and 303Se SStl variants [any hardness] are totally prohibited from MIL Acft uses. Based on further research, corrosion-pitting leads to intergranular attack leads to intergranular cracking and/or SCC [depending on static loading]... especially in high chloride environments. The peculiar nature of the 303 SStl alloy/chemistry, combined with residual forming stresses, appears to be the primary problem with these alloys.

The NSTB, SAIB and ATB reports appear to allow a ~15-year service limit for these parts... which is probably conservative.

NOTE. From the photos... and noted in the reports... the SStl safety wire and some 'odd shrink-plastic marker-tape'... wrapped around these terminal bodies appears to be the primary mechanism(s) for trapping moisture with chloride-contaminates against the terminal bodies. These 'traps' initiate pitting corrosion... that leads to intergranular attack and/or SCC. The 'bad deal' about the wrapped SStl wire and plastic tape is that there is no way for the contaminated moisture to 'evaporate/dissipate' which would allow the SStl surface to maintained contact with atmospheric oxygen for natural re-passivation. There is probably a 'oxygen depletion corrosion' initiation phenomena at play here.

Oddly, MIL-DTL-781 3.6.1 Surface treatment mandates that [1] of [4] optional surface treatments be applied: (a) passivation by one-of-3-methods [ASTM A967 or AMS2700]; or (b) glass bead peening per AMS2430 [which wasn't evident in photos]. Hmmmm.

As I see it...

The 303Se bar stock that these terminal swage-fittings are MF appears to be at the root of the problem. I am astonished that the specification even allows 303xx SStl in any form, based on its established 'bad actor' status in MIL-STD-1587... for many years.

IF these parts were MF 304 [or 304L or 316 or 321... 1/8H or even 1/4H] SStl bar, I doubt we would be seeing these failures.

NOTE. The MIL acft I've worked-on usually replace flight control cables on a 15-to-20 year cycle at depot maintenance. This replacement criteria usually has-to-do with the carbon steel wire-rope corroding and/or the breaking of wire filaments while constantly rolling-over under-sized [dia] pulleys [by today's standards].

Hope this makes sense. I need another 'cuppa' morning espresso.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: FAA AD on turnbuckle corrosion

(OP)
It is ironic that I can buy 316 or 316L turnbuckles for marine use , but not for aircraft usage .
May have to go back to bronze turnbuckles , if you can still get them.
B.E.

You are judged not by what you know, but by what you can do.

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